In recent years, with the increase of large display devices such as liquid crystal televisions, display devices have been actively developed. In specific, a technique of forming a driver circuit such as a gate driver over the same substrate as a pixel portion by using a transistor formed using a non-single-crystal semiconductor has been actively developed because the technique makes a great contribution for reduction in cost and improvement in reliability.
However, deterioration such as increase in threshold voltage or decrease in mobility is caused in the transistor formed using the non-single-crystal semiconductor. As the deterioration of the transistor advances, there is a problem in that the driver circuit becomes hard to operate and an image cannot be displayed. Accordingly, Patent Document 1 discloses a structure of a shift register which can suppress the deterioration of the transistor. In Patent Document 1, one electrode of a capacitor is connected to a wiring to which a clock signal is input and the other electrode of the capacitor is connected to gates of two transistors, so that the potential of the other electrode of the capacitor is increased or decreased by making the potential synchronize with the clock signal. In this manner, by utilizing capacitive coupling of the capacitor, signals that synchronize with the clock signal are generated in the gates of the two transistors. Then, by using the signals that synchronize with the clock signal, on and off of the transistors is controlled. Accordingly, since a period when the transistor is on and a period when the transistor is off are repeated, the deterioration of the transistors can be suppressed.